G. R. Foster

Revised Slope Steepness Factor for the Universal Soil Loss Equation

ABSTRACT Areanalysis of historical and recent data from both natural and simulated rainfall soil erosion plots has resulted in new slope steepness relationships for the Universal Soil Loss Equation. For long slopes on which both interrill and rill erosion occur, the relationships consist of two linear segments with a breakpoint at 9% slope. These relationships predict less erosion than current relationships on slopes steeper than 9% and slopes flatter than about 1%. A separate equation is proposed for the slope effect on short slopes where only interrill erosion is present. For conditions where surface flow over thaw-weakened soil dominates the erosion process, two relationships with a breakpoint at 9% slope are presented.

Revised Slope Length Factor for the Universal Soil Loss Equation

ABSTRACT An analysis based on theoretical considerations and data interpretation was used in developing revised relationships for the slope length exponent for the Universal Soil Loss Equation. The analysis was based on the ratio of rill to interrill erosion and resulted in a general relationship between the slope length exponent and slope steepness. Parameters in the relationship are changed depending upon whether the ratio of rill to interrill erosion is expected to be low, moderate or high. Such conditions might be representative of rangeland or no-till seeding, normal seedbed conditions or highly disturbed conditions, respectively. For thawing soil conditions where rill erosion is dominant, an exponent value of 0.5 is recommended..

A Runoff Erosivity Factor and Variable Slope Length Exponents for Soil Loss Estimates

ABSTRACT A runoff erosivity term was developed for the Universal Soil-Loss Equation (USLE) from analysis of an erosion equation derived from basic erosion principles. The analysis also suggested how the slope length exponent of the USLE varies with runoff, soil erodibility, slope steepness and length, and erosion control practice. The findings should improve USLE soil-loss estimates for specific events and time periods.

An Erosion Equation Derived from Basic Erosion Principles

ABSTRACT AN erosion equation is derived from the continuity equation for sediment transport and other equa-tions describing rill and interrill erosion. The resulting equation is a useful model for explaining the behavior of the erosion process. The equation might serve as the basis for an operational equation for estimating soil loss for specific storms.

Depositional Patterns of Sediment Trapped by Grass Hedges

Stiff-grass hedges can resist, retard, and disperse concentrated flows of runoff; trap suspended sediment; and reduce ephemeral gully development. Flume experiments were conducted at a 5% grade using several combinations of four grass species, four types of sediment, and eight flow rates ranging from 0.33 to 2.66 m3/min-m. Sediment trapping resulted primarily from deposition in the backwater upstream of the grass, rather than by filtration in the grass. Sediment was initially deposited 1 to 2 m downslope from a hydraulic jump transition that formed at the upper edge of the backwater. From this position, a delta of sediment mostly coarser than 125 mm grew back toward the hydraulic jump until flow depth became shallow enough that bedload transport was initiated and the delta began to advance toward the grass. In the flow zone across the delta, slope steepness approached 1 to 2% and the flow was near critical depth. A steady-state model was developed that describes sediment trapping in the backwater area as a settling process controlled by sediment characteristics, flow rate, and backwater depth at the grass. This model underpredicted trapping of fine sediment and overpredicted trapping of coarse sediment in situations where the delta neared the grass.

Sediment Composition for Nonpoint Source Pollution Analyses

ABSTRACT EQUATIONS were derived that describe the composition of sediment as a function of the distribution of primary particles in the matrix soil. These equations calculate the fraction of sediment in five particle classes—primary clay, primary silt, small aggregate, large aggregate, and primary sand—and the composition of the aggregate classes. The equations describe sediment at its point of detachment and are designed for use in nonpoint source pollution models that route sediment by particle classes to compute the transport of soil-associated contaminants from field-sized areas.

Estimating Sediment Transport Capacity in Watershed Modeling

ABSTRACT WATERSHED modelers are faced with a host of sed-iment transport formulas, each one derived for a limited range of sediment and streamflow characteris-tics. Predicted transport rates differ significantly from formula to formula, and selecting a reliable formula to predict a specific load is a difficult task. The authors have evaluated a number of transport formulas against laboratory and field data. The formulas were selected for their particular applicability to hydrologic modeling, and the data cover the wide range of parameters usually en-countered in agricultural watersheds. This paper presents a summary of the formulas and data used in the study, and the results of the evaluation.

Estimating erosion and sediment yield on field-sized areas

ABSTRACT A model for field-sized areas was developed to evaluate sediment yield under various management practices. The model provides a tool for evaluating sediment yield on a storm-by-storm basis for control of erosion and sedi-ment yield from farm fields. The model incorporates fun-damental principles of erosion, deposition, and sediment transport. The procedures allow parameter values to change along complex overland flow profiles and along waterways to represent both spatial variability and varia-tions that occur from storm to storm. Many of the model parameter values are obtained from topographic maps or directly from the Universal Soil-Loss Equation (USLE). Thus we feel that the model has immediate applications without extensive calibration. Individual components of the model were tested using experimental data from studies of overland flow, erodi-ble channels, and small impoundments. These results suggest that the model produces reasonable estimates of erosion, sediment transport, and deposition under a variety of conditions common to field-sized areas. The procedures developed here can be used to evaluate alter-native management practices such as conservation tillage, terracing, and contouring.

Evaluation of Rainfall-Runoff Erosivity Factors for Individual Storms

ABSTRACT THE Universal Soil-Loss Equation is widely used to satisfactorily estimate average annual soil loss from rainfall. Without modification, it is somewhat unsatis-factory for estimating soil loss from individual storms. Several erosivity factors that could be used to estimate soil loss from individual storms were investigated in this study. Rainfall erosivity factors like EI30 that include both rainfall rate and amount terms were found to be much better predictors of soil loss than a factor including rainfall amount alone. Lumped erosivity factors that in-clude rainfall amount, rainfall intensity, and runoff amount were even better predictors than EI30, whereas erosivity factors with separate terms for rainfall and runoff erosivity were best. However, these factors did not greatly improve soil loss predictions as compared with EI30.

Hydraulics of Failure of Unanchored Cornstalk and Wheat Straw Mulches for Erosion Control

ABSTRACT BASIC hydraulic theory that divides a flows total shear stress into that acting on soil and that acting on mulch was used to derive equations giving critical slope length for failure of unanchored mulch on untitled soils as a function of mulch type (cornstalk or wheat straw) and amount, slope, rainfall erosivity, runoff, and soil susceptibility to rill erosion. Parameter values for the equations were determined from experimental data. Values computed from the equations agreed well with recommended values for wheat straw mulch on construc-tion sites for slopes greater than 13 percent. Graphical solutions to the equations were prepared to facilitate ap-plication of the analysis.